The present invention relates to articles made from elastomeric material, particularly pneumatic tires, reinforced with rubberized fabrics comprising cords with at least one shape memory wire; and also to the said fabrics and to the corresponding cords.
The invention also relates to a process for the manufacture of these rubberized fabrics.
Many articles made from elastomeric materials, including pneumatic tires for vehicle wheels, conveyor belts, flexible hoses for the transport of fluids and similar, comprise at least one rubberized fabric formed by a plurality of reinforcing cords, normally textile or metal, disposed parallel to each other and incorporated in an elastomeric material.
In the following part of the present description, the wording xe2x80x9celastomeric materialxe2x80x9d is intended to denote the composition of the incorporating material as a whole, in other words the rubber, including the polymer base, the reinforcing fillers and the various protective, accelerating, anti-ageing and other agents, the whole according to recipes which are well known to those skilled in the art.
It is also known that metal cords are formed from a plurality of single metal wires wound spirally with respect to each other, with predetermined intervals, according to a plurality of configurations which are well known to those skilled in the art.
In general, the cited articles require cords having particular characteristics of mechanical strength when exposed to various stresses, including tensile and compressive stresses, and having corrosion resistance. Corrosion may be initiated in the metal wires of the cord by the presence of moisture in the residual air inside the cords incorporated in the rubber, or by direct contact with water when the breaking of the rubber layer exposes the cord to the external environment.
Once initiated, the corrosion may be propagated along the wires in the absence of a suitable protective coating of the wires.
To meet the requirement of corrosion resistance, it is convenient for the space between the metal wires of the cord to be completely filled with rubber to avoid the presence of air incorporated between the wires and subsequent formation of moisture with consequent development and propagation of the corrosion phenomenon.
Additionally, in order to resist mechanical stresses, the wires of the cords must be closely associated with each other in order to ensure correct behaviour in operation, as represented graphically, in a Cartesian stress-strain diagram, by a substantially linear characteristic.
In fact, due to the distance between the wires, a cord is subject to mechanical hysteresis and to a risk of failure of the wires; even under a compressive load lower than that withstood by a cord in which the distance between the constituent wires is minimal or zero.
The requirements of good penetration of the rubber between the wires and high performance of the cords in operation are particularly important in pneumatic tires; these are normally made by assembling a plurality of different semi-finished components, some of which consist of strips of various sizes formed from the previously cited rubberized fabrics.
The manufacture of the rubberized fabrics for pneumatic tires is carried out by incorporating the bare cords in an elastomeric material, preferably by means of known rubberizing devices, such as extruders and calenders, supplied from feed reels of the bare cords disposed before the said devices. It is during-this stage of incorporation that the penetration of the elastomeric material into the cords has to be achieved.
There are various known solutions designed to ensure good penetration of the rubber into the cord, all characterized in that the cords which are easily penetrable by the rubber do not have optimal behaviour in the pneumatic tire during its use.
In one solution suitable for stranded cords, the cord comprises a first pair of wires disposed in one plane and a second pair of wires disposed in a further plane which rotates with respect to the first along the longitudinal development of the cord, so that in each cross section the surfaces of the wires have maximum exposure and consequently maximum coating with elastomeric material. This solution entails a non-uniformity in the disposition of the wires along the development of the cord, with unsatisfactory performance in use.
A different solution specifies cords in which the wires are kept slack (open cords) so that a small distance is left between them. In the passage through the rubberizing device, the distance set between the wires permits good penetration of rubber into the cord. This solution may cause the compacting of the wires against each other, owing to the tension to which they are subjected even before they reach the device, thus making it impossible or very difficult for the rubber to penetrate into the cord; when this does not happen, the cord is rubberized in an optimal way but maintains a behaviour which is hysteretic, and therefore unsatisfactory, in use.
A further solution specifies the disposition in the cord of a wire having a non-linear (zigzag) configuration, so that a space is provided between each of the various wires and the next, and the penetration of rubber to the centre of the cord is promoted. This solution entails lower fatigue resistance of the non-linear wire and therefore of the whole cord.
If we now examine cords of the multilayer type, these comprise a central core covered with a plurality of concentric layers of wires, as in the case of the known cord having a 3+9+15 configuration, in other words a core of three wires twisted together, round which is wound a first layer of nine wires on which is wound a second layer of fifteen wires. These cords are used, in particular, in the casing plies of pneumatic tires for trucks.
In this cord, little rubber penetrates into the inner layer, and practically none penetrates into the core, owing to the physical barrier created by the radially outer layers of wires. In these types of cord, in order to achieve sufficient rubber penetration, the solution based on the use of wires of different diameters is convenient.
Although on the one hand this solution improves the rubber penetration, on the other hand it is unsatisfactory in respect of the performance of the cord in use.
To improve the characteristics of the behaviour of the pneumatic tyre in use, metal cords in which at least one of the component wires is made from an alloy of a shape memory material have recently been used.
Shape memory materials are described, for example, in pages 3 to 20 of the publication xe2x80x9cEngineering Aspects of shape memory alloysxe2x80x9d, Butterworth-Heinemann, published in 1990.
Shape memory wire, as will be described in greater detail subsequently, has the properties (1) of possessing a precise memorized shape which is imparted to it by a heat treatment carried out at a specified temperature which imparts to the wire a predetermined critical point, (2) of losing this shape as a result of mechanical stresses imparted at a temperature below the critical point, and (3) of returning to the memorized shape whenever its temperature exceeds the critical point.
For use in pneumatic tires, this type of wire, which has been heat treated so that it has, for example, an undulating shape, is subjected to a stretch which imparts another configuration, for example linear, at ambient temperature, before it is stranded with the other wires to form a cord.
Whenever the temperature in the pneumatic tire increases, for example as a result of high speed, to a point higher than the critical point of the shape memory wire, the wire tends to return to the originally memorized undulating shape.
However, since the shape memory wire is stranded with the other wires and the whole cord is fixed to the elastomeric matrix, and the whole structure is subject to tension, this wire is unable to contract to assume its own undulating configuration of lesser length.
Consequently, there is an increase in tensile stress in the shape memory wire (the wire acts as a stretched spring), the effect of which is to increase the rigidity of the structure in opposition to the effect of centrifugal force.
In particular, U.S. Pat. No. 5,242,002 describes a radial tire whose belt assembly comprises three belts, the first two having cords symmetrically inclined with respect to the equatorial plane and the third having cords disposed circumferentially.
The cords are formed from a plurality of wires wound spirally with respect to each other and each cord of the inner belts comprises a plurality of metal wires, at least one of which is made from an alloy of a shape memory material.
Japanese patent application JP 4362401 relates to a radial tyre having a belt structure whose outer layer comprises a two-way shape memory expansion element, preferably an element of the spring type made from a Ni-Ti alloy, wound in the circumferential direction (at 0xc2x0) on the underlying belt layers.
The shape memory element tends to contract in the circumferential direction when the tyre is subjected to heating in high speed travel. However, since this contraction is impeded by the underlying belt structure, the element develops a tensile force which makes the belt assembly more rigid, thus controlling the phenomenon of expansion of the tyre at high speed.
At low speeds or in normal conditions of use, the shape memory element maintains the initial shape or returns to the initial shape as a result of the inflation pressure. The applicant has perceived that the failure to achieve optimal behaviour as described above may depend on the particular behaviour of the said cords with shape memory wires which, together with their advantages, pose a considerable problem.
What happens in practice is that, during the vulcanization of the tyre, which, as is well known, is carried out at a temperature of the order of 150xc2x0 C. and sometimes above, in its initial stage, when the rubber compound has low viscosity, the contraction of the shape memory wire causes the opening of the cord, in other words the spacing apart of the component wires.
The rubber is then vulcanized, losing its plasticity, but the cord is unable to close up, being prevented from doing so by the contraction of the shape memory wire, and is therefore consolidated in the vulcanized tyre in this swollen configuration, with all the cited disadvantages of unstable behaviour and low compressive strength, resulting particularly in poor resistance to the bending and compression stresses.
The cited patents U.S. Pat. No. 5,242,002 and JP 4362401 fail to deal with this aspect, and therefore the problem of improving the penetration of the elastomeric material between the wires of a cord while obtaining good performance of the cord, and consequently of the tyre in use, remains substantially unresolved at the present time.
The applicant has realized that it is possible to improve simultaneously the characteristics of penetration of the rubber between the wires of a cord and the performance of the cord in the tyre in use, by making use of cords which contain at least one shape memory wire with characteristics of recovering a previously memorized shape, and are active principally in a first heat cycle, the wire also being provided with programmed significant characteristics of degradation of the memory after the first heat cycle.
The following preliminary observations and definitions relating to shape memory materials will help to provide a clearer understanding of the nature of the applicant""s invention.
Shape memory is the capacity, possessed by some metal alloys, of eliminating deformations of an apparently plastic nature by a suitable heating of the material.
It is known (xe2x80x9cShape Memory Alloysxe2x80x9d- ed. H. Funakubo - Gordon and Breach Science Publisher - 1987) that the properties of shape memory are imparted by a solid-solid phase transformation (from martensite to austenite when passing from low to high temperature, and vice versa), called xe2x80x9cthermoelastic martensitic transformationxe2x80x9d. This transformation is known as xe2x80x9cdirectxe2x80x9d in the case of cooling and xe2x80x9cinversexe2x80x9d in the case of heating. Direct transformation, which corresponds to the formation of the martensitic structure, starts at a temperature Ms and finishes at a lower temperature Mf. Inverse transformation, which corresponds to the formation of the austenitic structure, starts at a temperature As and ends at a higher temperature Af.
Since, in general, Msxe2x89xa0Mfxe2x89xa0Asxe2x89xa0Af, the said martensitic transformation is hysteretic. In particular, if Mf less than Ms less than As less than Af, the martensitic transformation is said to be of Type 1; if Mf less than As less than Ms less than Af, the martensitic transformation is said to be of Type 2.
The martensite phase has a typical microstructure consisting of dominoes (called martensitic variants) which may be orientated differently under the action of even limited stress states (e.g. 50 MPa). A shape memory material acquires a predetermined shape by a heat treatment for a specific time and at a specific temperature. This treatment is carried out on the wire of a specific material of particular composition in order to obtain a predetermined transformation temperature. When the material is cooled, the transformation from the austenite phase to the martensite phase takes place, and, if the material is subjected to a stress state capable of producing the process of orientation of the variants, the deformation xcex5* associated with this phenomenon, becomes permanent, for temperatures of less than As, after the removal of the force (pseudo-plastic deformation). However, during the subsequent heating to temperatures of more than As, the deformation xcex5* is eliminated by inverse martensitic transformation, and consequently the original shape is recovered (the shape memory effect). The elimination of the deformation xcex5* is total if xcex5*  less than /=xcex5max where xcex5max is the maximum deformation eliminable by the shape memory effect, and is characteristic of the specific shape memory material and of the specific heat treatment used to impart the memory. If the elimination of xcex5* is impeded, partly or entirely, by conditions of mechanical constraint in the passage from the temperature As to the higher temperature Af during heating, the material develops a tensile force called the reconversion force.
In conclusion, the heat treatment is used to impart the four characteristic temperatures of a shape memory alloy, indicated above as Ms, Mf, As, Af.
The capacity of complete elimination of the deformation xcex5* in the subsequent heat cycles undergone by the material is generally subject to a degradation, represented by the decrease in the number of subsequent heat cycles in which this elimination can be obtained, this degradation increasing as xcex5* approaches xcex5max. The decrease in the value of the portion xcex5* of the residual eliminable pseudo-plastic deformation, also known as the xe2x80x9cshape memory degradationxe2x80x9d, is defined as a continuous change of the characteristics of the shape memory of a material, determined by the number of heat cycles undergone, and represents the useful life of a shape memory material.
For a more precise definition of the shape memory degradation of a material, reference should be made to the description in pages 256 to 259 of the publication xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9d, Butterworth-Heinemann, published in 1990. In this publication it is stated that the life of such a material is expressed as the recoverability of a given previously memorized shape. When the material is no longer capable of recovering the memorized shape, its useful life is considered to be ended.
For example, for a NiTi alloy in which xcex5max=8%, the number of subsequent heat cycles for which a deformation xcex5* can be repeatedly and completely eliminated varies as a function of the value of xcex5*, as shown in the following table (from J. Cederstrom and J. VanHumbeeck, J. de Physique IV C2, 1995, pp. 335-341).
It will be seen from the table that if an elongation xcex5* (pseudo-plastic deformation) of 8% is imparted to the material, particularly to the metal wire, it will be completely eliminable during the first heat cycle, but will no longer be eliminable in the subsequent heat cycles, during which only a progressively decreasing fraction of this elongation can be eliminated. Conversely, if the imparted pseudo-plastic elongation xcex5* is only equal to 2%, it will be completely eliminable through 10000 subsequent heat cycles before the start of degradation. For the purposes of the present invention, each heat cycle comprises both the heating phase and the subsequent phase of cooling of the material.
If a pseudo-plastic deformation xcex5tot of more than xcex5max is imparted to the said material, this deformation consists of an eliminable portion xcex5* and a non-eliminable portion xcex5pl (plastic deformation). Therefore xcex5tot=xcex5*+xcex5pl.
In this case also, in subsequent heat cycles xcex5* always coincides with xcex5max, although here the value of xcex5max changes continuously and in each specific cycle depends on the number of heat cycles undergone previously.
In other words, if the same deformation xcex5tot is always produced at the end of each heat cycle, the composition of xcex5tot varies from one cycle to the next, with a progressive decrease in the eliminable portion xcex5* and a simultaneous increase in the portion of plastic deformation xcex5pl.
The applicant has realized that considerable advantages in the performances of cords can be obtained by using, for at least one wire, shape memory materials with suitable characteristics of memory degradation produced in the wire by a specific heat treatment carried out on the wire before it is stranded with the other wires.
The applicant has realized that it is possible to make advantageous use of the shape memory effect of the wire, in other words the capacity of eliminating an imposed elongation by the recovery of a predetermined initial shape, by limiting this effect to the phase of incorporation of the cords in an elastomeric material, in order to obtain optimal penetration of the rubber into the cord, making this phase simultaneous with the first heat cycle to which the cord, and with it the shape memory wire, is subjected.
Preferably, this phase of incorporation is carried out at a temperature T1 which is greater than the minimum temperature As of the transformation range [As-Af] assigned to the wire and, even more preferably, also greater than the maximum temperature Af of the said range.
The shape memory wire is previously subjected to an elongation of predetermined value xcex5* while it is at a temperature T0 lower than As (for example, ambient temperature), and is then stranded together with the other wires, by known methods and means, to form a cord.
In the phase of incorporation of the cord which contains the said shape memory wire, carried out at high temperature, the elimination of the deformation takes place in association with a contraction of the wire which, in a condition of friction with the other wires of the cord, develops a contractile force and therefore causes a disarrangement of the wires, in other words a swelling of the cord.
In practice, the cord is made to open with consequent good penetration of rubber into it.
Subsequently, the tension exerted on the cords after the incorporation phase, during the picking up of the fabric and its cooling from the incorporation temperature to values progressively decreasing to the ambient temperature, advantageously causes the recovery of the deformation state of the shape memory wire with a return to the value of xcex5*, possibly by means of the limited forces required by the processes of orientation of the martensite, with a consequent return of the wires towards each other in the cord, until their compacting, in other words the closing of the cord, is obtained.
This compact configuration is maintained practically unchanged in the subsequent heat cycles owing to the characteristics of degradation of the shape memory imparted to the shape memory wires which make it impossible to recover a substantial portion of xcex5*.
In this way the maintenance of a substantially closed configuration of the cords in the subsequent vulcanization heat cycle is obtained, despite the high temperature of the cycle, so that the cord becomes incorporated in the vulcanized tyre in a substantially closed configuration.
Consequently, articles, and in particular pneumatic tires, constructed with rubberized fabrics prepared as stated above show optimal performance of the cords.
In a first aspect, the invention therefore relates to a metal cord for reinforcing articles made from elastomeric material, comprising a plurality of metal wires wound spirally around each other, at least one of which is formed from a shape memory material, is able to recover a previously memorized shape and is deformed away from the said memorized shape, the said cord being characterized in that the said shape memory wire has the said recovery capacities substantially active in a first heat cycle and degraded to at least a significant predetermined extent after the said first heat cycle.
In another aspect, the invention relates to a metal cord for reinforcing articles made from elastomeric material, such as pneumatic tires, conveyor belts, flexible hoses and similar, comprising a plurality of metal wires wound spirally around each other, at least one of the said wires being formed by a shape memory material, the said cord being characterized in that the said shape memory wire, at ambient temperature, has:
the memory of a different shape, with a length l0 which is less than the length l1 of the wire at ambient temperature, memorized at a temperature As which is greater than the ambient temperature T0;
a pseudo-plastic elongation xcex5max/c eliminable by the shape memory effect, and having a value of between 0.2% and 8% of the length of the said memorized shape;
an elongation xcex5tot having a value of at least 85% of the said value xcex5max/c;
a decrease in the residual eliminable pseudo-plastic elongation xcex5*, after a first heat cycle carried out at a temperature T1 greater than As, this decrease being at least 40% of the value of the said pseudo-plastic elongation xcex5max/c.
In a second aspect, the invention relates to a rubberized fabric for use in articles made from elastomeric material reinforced with the cords according to the invention, as defined above: alternatively, the invention relates to a rubberized fabric for use in articles made from elastomeric material comprising a plurality of reinforcing cords incorporated in the elastomeric material of the said fabric and disposed so that they are coplanar with, parallel to and adjacent to each other in the same direction, each cord being formed by a plurality of metal wires wound together spirally, at least one of the constituent wires of at least one of the said cords being formed from a shape memory material, the said fabric being characterized in that the said wire made from shape memory material has the following characteristics at ambient temperature:
the memory of a different shape, with a length l0 which is less than the length l1 of the wire at ambient temperature, memorized at a temperature As which is greater than the ambient temperature T0;
a pseudo-plastic elongation xcex5max/t eliminable by the shape memory effect, and having a value of between 0.1% and 8% of the length l0 of the said memorized shape;
a pseudo-plastic elongation xcex5tot having a value of at least twice the said value xcex5max/t;
a decrease in the residual eliminable pseudo-plastic elongation xcex5*N+1 for each subsequent heat cycle, carried out at a temperature T1 greater than As, this decrease being at least 40% of the value of the pseudo-plastic elongation xcex5max/N of the preceding cycle.
In the fabric according to the invention, the perfect rubberizing of the metal wires of the cords was obtained during the fabric rubberizing heat cycle by the spacing actions exerted on the adjacent metal wires by the shape memory wire which tends to recover the predetermined memorized shape of smaller length, with consequent renewed swelling of the cord and penetration of rubber between the wires of the open cord: conversely, the good performances of the cords of the said fabrics in the tyre in use are obtained by the configuration of the cords which remains substantially closed in the heat cycles developed during the use of the tyre, owing to the decrease in the value of the residual pseudo-plastic elongation xcex5* eliminable by the shape memory effect, this decrease occurring as a result of the heat cycles of the rubberizing of the fabric and the vulcanization of the tyre.
In a third aspect, the invention relates to an article made from elastomeric material, and more particularly to a pneumatic tyre for vehicle wheels, reinforced with the cords according to the invention, and more preferably with the rubberized fabrics according to the invention, as described above; in a preferential aspect, the invention relates to a pneumatic tyre for vehicle wheels, comprising a toroidal casing having a crown portion and two axially opposing sides, terminating in a pair of beads for fixing the tyre to a corresponding mounting rim, a tread band disposed on the crown of the said casing and a belt structure interposed between the said casing and the said tread band, the structure of the said tyre comprising a plurality of reinforcing cords, each formed by metal wires wound spirally with respect to each other, at least one of which is a wire made from a shape memory material, characterized in that the said wire made from a shape memory material has the following characteristics are ambient temperature:
the memory of a different shape, with a length l0 which is less than the length l1 of the wire at ambient temperature, memorized at a temperature As which is greater than the ambient temperature T0;
a pseudo-plastic elongation xcex5max/p eliminable by the shape memory effect, with a value of between 0.05% and 8% of the length l0 of the said memorized shape;
a pseudo-plastic elongation xcex5tot having a value of at least six times the said value xcex5max/p;
a decrease in the value of the residual eliminable pseudo-plastic elongation xcex5*N+1 for each heat cycle following that of the vulcanization of the tyre, carried out at a temperature T1 greater than As, this decrease being at least 40% of the value of the pseudo-plastic elongation xcex5max/N of the preceding cycle.
Preferably, the tyre is of the radial type and the rubberized fabrics with cords comprising at least one shape memory wire are used in the belts and/or in the plies of the casing.
In a further aspect, the invention also relates to the process of assembly of the said pneumatic tire, characterized by the use of the cords as described above.
In yet another different aspect, the invention relates to a process for the manufacture of a rubberized reinforcing fabric for articles made from elastomeric material, such as pneumatic tires conveyor belts, flexible tubes and similar, comprising a plurality of reinforcing cords oriented parallel to each other in a single direction and incorporated in the elastomeric material of the said fabric.
In these fabrics, each cord comprises metal wires wound spirally around each other and, in at least one of the said cords, at least one of the component wires is formed from a shape memory material which has memorized, by means of a suitable heat treatment, a predetermined shape with a length less than that of the wire at ambient temperature and which is deformed by elongation at ambient temperature by a predetermined percentage amount xcex5tot.
The process, comprising the known phases of incorporating the cords in a layer of elastomeric material to form the said reinforcing fabric, and then cooling and picking up the fabric, is based on the innovative phases of:
a) using a shape memory wire with characteristics of degradation of the shape memory effect such that the pseudo-plastic elongation xcex5max eliminable by the shape memory effect, after the heat cycle of the rubberizing of the fabric, lies between a value of zero and a value equal to a maximum of 40% of the initial value xcex5max, with a decrease in xcex5max in each subsequent heat cycle preferably having the same percentage value as that in the preceding cycle;
b) incorporating the cords in the elastomeric material at a temperature T1 greater than the temperature of the start of the transformation phase As;
c) in the phase of incorporation of the cords in the elastomeric material, using the recovery of the predetermined shape memorized by the wire to transmit to the surrounding wires the reconversion force originating during the said recovery, with effects of spacing the said wires away from each other and penetration of the rubber into the cord in a substantially open configuration;
d) pulling the cords during the cooling and pick-up of the fabric to restore the original length of the said cords.